There are two types of chromatic dispersion devices in which a chromatic dispersion value varies almost linearly according to a wavelength; one is an all-pass resonator Fabry-Perot etalon (FPE) type (see, M. Jablonski, et al., “Adjustable dispersion-slope compensator using entirely thin-film coupled-cavity all pass filters in a multi-reflection parallel configuration,” Optical Fiber Communication Conference, Anaheim, Calif., 2001, TuS3; and D. J. Moss, et al., “Multichannel Tunable Dispersion Compensation Using All-Pass Multicavity Etalons,” Optical Fiber Communication Conference, Anaheim, Calif., 2002, TuT2), and the other type is ring resonator type (see C. K. Madsen, et al., “Compact Integrated Tunable Chromatic Dispersion Compensator with a 4000 ps/nm Tuning Range,” Optical Fiber Communication Conference, Anaheim, Calif., 2001, PD9). Both types of the chromatic dispersion devices can compensate chromatic dispersion of a plurality of wavelengths in a lump and suitable for wavelength division multiplexing (WDM) transmission. A ring resonator type chromatic dispersion device, in particular, has a wide variable range of ±1000 ps/nm or more.
FIG. 9 shows a schematic diagram of a conventional ring resonator type chromatic dispersion device. In FIG. 9, a chromatic dispersion device 110 comprises a linear input/output waveguide 112 and a plurality of ring waveguides 114, 116, 118, and 120 disposed along an optical axis of the input/output waveguide 112 and directionally coupled with the waveguide 112. Free spectral ranges (FSR) of the ring waveguides 114 to 120 are set equally.
A part of light entered the input/output waveguide 112 couples with the waveguide 114 at the optically coupled part with the ring waveguide 114. The light coupled with the ring waveguide 114 makes one circuit of the ring waveguide 114 and couples with the input/output waveguide 112 again. A similar operation is also carried out in the other ring waveguides 116, 118, and 120. Input/output characteristics of the chromatic dispersion device 110 are obtained by multiplying input/output characteristics of the respective ring waveguides 114 to 120. By adjusting coupling factor between each of the ring waveguides 114 to 120 and the waveguide 112, group delay characteristics of the respective ring waveguides 114 to 120 can be controlled.
FIG. 10 shows group delay characteristics 124 to 130 of the ring waveguides 114 to 120 respectively and total group delay characteristics 132. In FIG. 10, the horizontal axis expresses relative angular frequency (Ω–Ω0)/FSR, and the vertical axis expresses relative group delay τ(Ω)/T. The symbol Ω denotes angular frequency of a light input to the optical waveguide 112, Ω0 denotes a standard angular frequency, T denotes a standard delay time (second), and τ(Ω) denotes group delay time of the chromatic dispersion device 110. As shown in FIG. 10, the group delay time τ(Ω) of the chromatic dispersion device 110 ranges from −π/2 to a value slightly lower than π and varies almost linearly relative to the angular frequency Co. This range becomes a band usable for chromatic dispersion compensation, namely passband 134. The range removed the passband 134 from the FSR indicates an unusable band 136.
In a conventional ring resonator type chromatic dispersion device, a group delay of each ring waveguide indicates a symmetrical resonant peak shape and each peak has the same sign (the positive sign in FIG. 10). As a result, the composite characteristics inevitably go off the straight line at the bottom part and therefore the unusable band 136 is appeared as shown in FIG. 10.